Photoelectrochemical mineralization of textile diazo dye pollutants using nanocrystalline WO3 electrodes

Photoelectrochemical degradation processes of Naphthol Blue Black (NBB) dye in aqueous solutions were investigated seeking an effective method of degrading textile diazo dyes. Nanocrystalline WO3 film on a Pt mesh electrode was used as the photoanode for photoelectrochemical degradation of NBB dye. Higher rates of NBB degradation were observed in chloride-containing environments than in sulfate solutions. A strong pH dependence of the NBB dye degradation rate has been found in chloride media. The pH dependence follows the trend of decreasing oxidizing power of electron holes, h(VB,s), trapped at the surface of WO3 particles, with increasing solution pH. In the low pH range (1.0-4.5), photooxidation of the medium favors the formation of Cl-. and Cl-2(.-) radicals which are powerful oxidants for the NBB dye. In alkaline solutions, the formation of OH. radicals is predominant over Cl-. and Cl-2(.-) radicals but the kinetics of NBB degradation is much slower than in acidic media. In sulfate solutions, the rate of NBB photodegradation is slower than in chloride media. Evidently, the lower oxidizing power of the dipersulfate ion S2O8-2 being the oxidant in this environment, is the limiting factor. The intermediates of the NBB dye degradation were detected using HPLC analysis. Possible dye degradation mechanism is proposed. (C) 2001 Elsevier Science Ltd. All rights reserved.